diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -971,6 +971,9 @@
 coreconfigitem('ui', 'debugger',
     default=None,
 )
+coreconfigitem('ui', 'debugtimestampfmt',
+    default=None,
+)
 coreconfigitem('ui', 'editor',
     default=dynamicdefault,
 )
diff --git a/mercurial/ui.py b/mercurial/ui.py
--- a/mercurial/ui.py
+++ b/mercurial/ui.py
@@ -9,6 +9,7 @@
 
 import collections
 import contextlib
+import datetime
 import errno
 import getpass
 import inspect
@@ -440,6 +441,8 @@
                 "report_untrusted")
             self.tracebackflag = self.configbool('ui', 'traceback')
             self.logblockedtimes = self.configbool('ui', 'logblockedtimes')
+            self.debugtimestampfmt = self.config('ui', 'debugtimestampfmt',
+                                                 None)
 
         if section in (None, 'trusted'):
             # update trust information
@@ -1394,6 +1397,9 @@
         '''
         if self.debugflag:
             opts[r'label'] = opts.get(r'label', '') + ' ui.debug'
+            if self.debugtimestampfmt:
+                now = datetime.datetime.now()
+                self.write(now.strftime(self.debugtimestampfmt), ' ', **opts)
             self.write(*msg, **opts)
 
     def edit(self, text, user, extra=None, editform=None, pending=None,